This invention relates generally to heat removal in computer systems, and more specifically, to an improved heat removal device for mobile computing systems.
As mobile computing systems (e.g., laptops) become smaller and smaller, the need for design flexibility increases. The power level of laptop processors is increasing with a corresponding increase in heat that must be removed from the system.
FIGS. 1A and 1B show a front view and a side view, respectively, of a typical heat transfer system used in mobile computing applications. The heat transfer system 100A shown in FIG. 1A, includes a substrate 102A with a die 104A sitting on top of the substrate 102A. Die 104A is typically made of silicon and contains the electronic components of the microprocessor. Heat is generated in die 104A and passed through thermal interface material (TIM) 106A to a heat spreader 108A. Heat spreader 108A is typically larger than the die 104A. The TIM 106A reduces the contact resistance between the die 104A and the heat spreader 108A. The Tim 106A may be solder, a particle-laden polymer, or other material exhibiting similar thermal properties. Heat spreader 108A is typically a copper block and is soldered to heat pipe 112A with a solder layer 110A. Embedded in heat pipe 112A is a wick structure 114A and a vapor space 116A that contains vapor. The walls of the heat pipe are typically copper. The heat generated in the die 104A is used to heat the liquid in the vapor space to convert it to a vapor. The vapor then condenses when heat is drawn through the heat sink 118B depicted in FIG. 1B. The heat sink 118B is typically a copper or aluminum block that may have fins to dissipate the heat more quickly. The wick structure 114A works as a capillary pump that brings the condensed liquid back to the heating region thereby maintaining a continuous loop.
This cooling method is known as remote cooling because the heat is not ejected at the location of the die, but is transferred elsewhere and ejected. In a typical desktop computer the heat sink can be placed directly on top of the die, but for mobile applications a thinner implementation is desired. Another reason remote cooling is desired in mobile applications is that it allows for the heat sink to be located next to an exhaust fan typically located in a corner of the laptop. This allows the heat to be carried out of the mobile system quickly.
The prior art heat transfer system presents several problems concerning wick structure 114A. The first is due to the fabrication process used to create the wick structure 114A. Typically a wick structure is made of porous copper. The wick structure is fabricated by sprinkling powdered copper along the inner length of the heat pipe. The powdered copper is then heated and slightly melted. This forms a porous copper structure. This process is not exact, and the wick structure 114A typically has large variations in its thickness along the length of the heat pipe. Because the vapor space 116A is a space above the wick structure 114A, variations in the thickness of the wick structure 114A cause corresponding variations in the thickness of the vapor space.
The thermal resistance is inversely proportional to the 4th power of the vapor space thickness or radius. Therefore small variations in the thickness of the vapor space 116 cause large variations in the thermal resistance.
Another problem with the prior art heat transfer system 100A is in the component layout. Typically the fan is located in the corner and the processor is located somewhere else. Since it is desirable to have the heat sink next to the fan, the heat pipe may have to be twisted and bent to accommodate component layout. This twisting and bending can also lead to variations in the thickness of the wick structure and therefore variations in the vapor space.
Another drawback is that the current fabrication process provides one wick structure for all areas of the heat transfer process. Ideally, to enhance the performance of a heat pipe, it is desired to have wick structure with variable porosity so that the evaporative and the condenser section have highly porous wick structures to enhance the boiling and condensation heat transfer and the adiabatic section has a different wick structure for optimized pressure drop. The current manufacturing technology of heat pipes does not allow this.
Another problem with the heat pipe technology is that if the manufacturing process is not very controlled, there could be clogging of the vapor space due to variations in wick thickness. This will lead to a very poor thermal performance of the heat pipe.
Performance of current heat pipe technology also suffers from the variation in the weight of wick and in water charge level.